In a TDMA system, an adjacent cell is monitored during an idle slot in a communication frame by changing a frequency from a presently used channel to a channel which is considered to be appropriate in the adjacent cell, and the formerly used channel is restored by changing again the frequency. For this purpose, a technology in which the switch-over of oscillation frequencies is carried out with high speed is required in the TDMA system.
A conventional frequency synthesizer used in a TDMA system comprises a reference frequency oscillation unit for generating a reference frequency dependent on a carrier frequency, a voltage controlled oscillator for generating carrier frequency dependent on an applied control voltage, a frequency division circuit for dividing the carrier frequency by a fixed division ratio, a phase comparator for comparing the reference frequency and a divided carrier frequency to provide a phase difference therebetween, a charge pump for charging and discharging a capacitor by receiving the phase difference, and a loop filter having a capacitor to be charged by the charge pump for generating the control voltage to be applied to the voltage controlled oscillator, herein the voltage controlled oscillator, the frequency division circuit, the phase comparator, the charge pump and the loop filter are connected to provide a phase locked loop which is closed and opened by a loop on/off switch.
In operation, the loop on/off switch is turned on, and a power supply voltage is applied to the reference frequency oscillation unit, and the frequency division circuit and the phase comparator in the phase locked loop, so that the voltage controlled oscillator is applied with a control voltage from the loop filter having the capacitor which is charged and discharged dependent on the phase difference detected in the phase comparator by the charge pump. Thus, a controlled carrier frequency is generated in the voltage controlled oscillator. Then, the loop on/off switch is turned off to make the phase locked loop open, and the power supply voltage is not applied to the reference frequency oscillation unit, and the frequency division circuit and the phase comparator in the opened phase locked loop, so that the voltage controlled oscillator is maintained to generate a carrier frequency which is determined by an electric charge voltage of the capacitor in the loop filter. Thus, the phase locked loop is controlled to operate intermittently, thereby realizing the saving of electric power.
In the conventional frequency synthesizer use in a TDMA system, the reference frequency is changed dependent on a controlled carrier frequency obtained in the voltage controlled oscillator, so that the switch-over of channels having a small step width is carried out, while a frequency division ratio of the frequency division circuit is suppressed to be low. Consequently, the switch-over of frequencies is realized with high speed.
However, the conventional frequency synthesizer has a disadvantage in that the loop intermittent operation for the electric power saving is limited to be carried out during a period in which the switch-over of channels is not performed, because the turning-off of the loop on/off switch to provide the loop intermittent operation is difficult to be carried out subsequently to the voltage stabilization of the capacitor in the loop filter under the situation where the switch-over of channels is performed in a TDMA system by a short period. In more detail, the capacitor in the loop filter is frequently charged and discharged at a communication frame, during which synchronism is set up with a separated frequency for the switch-over of channels, by the charge pump. In this situation, the capacitor takes a time in realizing the voltage stabilization due to the occurrence of dielectric absorption current flowing in the capacitor immediately after the charge and discharge thereof.